This invention is an improvement in photographic printers used to make film or paper prints from film transparencies. The object of normal photographic printing is to duplicate, as nearly as possible, the subject matter contained in the original. However, there are also photographic applications in which a perfect duplicate is not desirable. Many scientific, military and topographic reproduction systems require a capability to modify the reproduction means in order to convey all of the information available in the original to the reproduction material. Normal contrast and tonality can not always be preserved since, for example, the light transmission range from the darkest to lightest points in an original photograph may be as great as 100 to 1 or more, while a conventional photographic paper can only reproduce a light reflection ratio of, perhaps, 20 to 1.
The present invention relates to a class of photographic printers which use cathode ray tube (CRT) printing light sources. Such printers alter the gross contrast ratio of the print to enable reproduction of all of the information contained in the camera negative. The gross (ie, large area compared to the size of the scanning spot) contrast ratio existing in the negative is altered electronically to preserve all of the detail and edge contrast gradations. This type of photographic print may have an appearance of "flatness" or unreality but does, in fact, include all of the information present in the camera original.
There are two basic types of CRT printers in the prior art: those that modulate the intensity of light generated by the CRT beam, and those which modulate the velocity of the scanning electron beam in the CRT.
Intensity modulated CRT printers alter the gross contrast of the print by maintaining the light transmitted through the transparency within a given, predetermined brightness range, which may be from 2 to 1 to 20 to 1. Frequently, such printers also include means to selectively alter the contrast ratio within preselected limits. In intensity modulated systems it is important that the velocity or rate of deflection of the scanning beam should remain essentially constant, since the resulting photographic exposure of any incremental area is equal to the product of intensity and time.
In velocity modulated printers, however, it has been customary to maintain the beam intensity constant since the scanning velocity is varied for contrast correction purposes.
Since most CRT printers are used in exacting scientific or military environments, substantial control precision is required. Hence, each separate "family" of such printers has sought to stabilize, as much as possible, at least one component of exposure, be it the scan velocity for intensity modulated (IM) systems or the beam intensity for velocity modulated (VM) systems.
Velocity modulated systems generally achieve adequate compression of the tonal range, but are not well suited to the provision of a preselected contrast control function.
The present invention relates to a new and improved printer that combines both intensity and velocity modulation methods. It provides an apparatus for producing uniform exposures during contact or projection printing.